STRUCTURAL INVERSION, IMBRICATE WEDGING AND OUT-OF-SEQUENCE THRUSTING IN THE SOUTHERN JUNGGAR PETROLEUM PROVINCE, NORTHERN TIAN SHAN, CHINA

The southern Junggar basin, NW China, is an active fold-and-thrust belt that represents the foreland of the northern Tian Shan. This region has been an active petroleum province since the 1930’s, but with limited production. Using seismic reflection data, surface geology, and balanced cross-sections, we present a detailed history of the timing, evolution, and structural style of this thrust belt. In addition, we discuss how the complex structural style and sequence of deformation in the basin has influenced charge and trap histories for the petroleum system.

The most prominent fold trend in southern Junggar consists of the Huoerguosi, Manas and Tugulu anticlines. Petroleum exploration in these anticlines target clastic Tertiary reservoirs charged from Jurassic source rocks. 2- and 3-D seismic data reveal several of the structures in southern Junggar overlie high-angle faults, which we interpret to be Jurassic rift basins that have since been tectonically inverted. A well-imaged angular unconformity in the Tian Shan range front constrains the time of inversion as late Jurassic. Subsequently, a series of detachment levels propagated northward from the range front into the foreland. Fault ramps that step up from these detachment levels have produced a series of imbricated fault-related folds and detachment anticlines across southern Junggar. These structures are internally complex, exhibiting evidence of coeval fore-thrusts and back-thrusts, which we interpret as structural wedges that cut the prominent reservoir sequences. This has led to segmented hydrocarbon reservoirs that have compartmentalized Eocene reservoir pressures and oil-water contacts. The uppermost thrust sheet of this thrust belt, the Southern Junggar Thrust (SJT), truncates the crests of the Huoerguosi-Manas-Tugulu structures and has deformed Quaternary fluvial terraces at the surface. Thus, the SJT represents an active, out-of-sequence thrust. Taken together, the southern Junggar basin has experienced a complex structural history. Our analysis of the timing and structural style of southern Junggar improves the characterization of potential charge pathways and trap geometries. Moreover, this work highlights the need for detailed and precise structural interpretations in order to limit exploration risks in fold-and-thrust belts.